Polarized Stereoscopic Projection System and Method

ABSTRACT

A stereoscopic display system and method of stereoscopic projection that include a polarization-switching light source and a polarization-preserving projector. The polarization state of the polarization-switching light source is synchronized with alternate projection of left-eye images and right-eye images. The polarization-switching light source may include a laser and a rotating disk, and the disk may include a waveplate that switches the polarization state as the disk rotates.

BACKGROUND OF THE INVENTION

Stereoscopic display systems may be formed by using polarized light such that one polarization state is used for the left-eye image and the orthogonal polarization is used for the right-eye image. Glasses with polarizing filters are used to allow only the left-eye image to enter the left eye of the viewer and only the right-eye image to enter the right eye of the viewer. Stereoscopic projectors generally include spatial light modulators (SLMs) which act as planar light valves with individual elements that turn on and off to form corresponding individual pixels of a projected digital image.

SUMMARY OF THE INVENTION

In general, in one aspect, a stereoscopic display system including a polarization-switching light source and a polarization-preserving projector which is illuminated by the polarization-switching light source.

Implementations may include one or more of the following features. The polarization-preserving projector may form a left-eye digital image and a right-eye digital image, and the polarization state of the polarization switching light source may be changed in synchronization with an alternating projection of the left-eye digital image and the right-eye digital image. The alternating projection of the left-eye digital image and the right-eye digital image may form a moving image, and the polarization state of the left-eye digital image may be orthogonal to the polarization state of the right-eye digital image. There may be a polarization-preserving spatial light modulator in the projector, and the polarization-preserving spatial light modulator may be a digital micromirror device. There may be a Philips prism in the projector and the Philips prism may include a color-splitting filter with low polarization splitting. The polarization-switching light source may include a laser, and may include a rotating disk or an electro-optical device that switches the polarization state. The rotating disk may include a wave plate. All the components of the polarization-preserving projector may be polarization-preserving components. The polarization-preserving projector may be a front projector. There may be a polarization-preserving screen.

In general, in one aspect, a stereoscopic display system including a polarization-switching light source, a projector which is illuminated by the polarization-switching light source, and a polarization compensation element. The polarization compensation element compensates for polarization change in the projector.

In general, in one aspect, a method of stereoscopic projection including the steps of generating light which is polarization switched, modulating the light to make left-eye and right-eye digital images, and projecting the left-eye and right-eye digital images alternately to form projected left-eye and right-eye digital images.

Implementations may include one or more of the following features. The modulating may be synchronized with the polarization state of the light. The projected left-eye digital image may have one polarization state, and the projected right-eye digital image may have the orthogonal polarization state. The light may be laser light.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a side view of a stereoscopic display system;

FIG. 2 is a side view of a stereoscopic display system with a polarization-switching light source;

FIG. 3 is a top view of a polarization-preserving projector;

FIG. 4 is a side view of a polarization-switching light source;

FIG. 5 is a front view of a rotating disk in a polarization switch; and

FIG. 6 is a flowchart of a method of stereoscopic projection.

DETAILED DESCRIPTION

Polarized light may be used to form stereoscopic images when there are two orthogonal states of polarization. Orthogonal polarization states mean that two different orientation or types of polarizing filters are able to fully and distinctly separate the two polarization states without overlap. For example, linearly polarized light with an electric field vector oscillating in the horizontal direction is orthogonal to linearly polarized light with the electric field vector oscillating in the vertical direction. Similarly, linearly polarized light with an electric field vector oscillating in the −45 degree direction is orthogonal to linearly polarized light with the electric field vector oscillating in the +45 degree direction. Also, circularly polarized light with an electric field vector rotating in the clockwise (right-hand) direction is orthogonal to circularly polarized light with an electric field vector rotating in the counter-clockwise (left-hand) direction. If two states of non-orthogonal polarized light are used for stereoscopic viewing, the left eye image will leak into the right eye and vice versa to make ghosting artifacts which detract from the quality of the stereoscopic viewing experience.

Circularly polarized light is commonly used for stereoscopic projection because viewer head tilt does not significantly change the viewing experience. In contrast, when using linearly polarized light, severe ghosting artifacts will appear if the viewer's head is tilted too far. Also if the projector alone produces linearly polarized light, an external device may be placed in front of the projector to rapidly change the polarization between left-hand and right-hand circular polarization states. The external device may be an electrically-controlled liquid-crystal polarization rotator which is driven in synchronization with the projection of left-eye images and right-eye images.

By alternating the polarization states of left-eye images and right-eye images, stereoscopic content may be displayed and viewed. By rapidly alternating the polarization and images at faster than the flicker-fusion frequency, the appearance of moving images may be obtained.

Polarization preserving optical elements are used to transmit or reflect light without changing its polarization state. If two orthogonal polarization states are transmitted and reflected multiple times by the optical elements in a polarization preserving optical system, the two orthogonal polarization states are maintained in the original two orthogonal polarization states. Birefringence is the property of optical elements that describes the case where different directions have different indices of refraction. Birefringence generally leads to changes in polarization state, so low birefringence is usually desirable when designing polarization preserving optical systems.

FIG. 1 shows a stereoscopic display system. Light source 100 illuminates projector 102. Projector 102 projects an image with lens 104. Projector 102 and lens 104 illuminate polarizing filter 106 which polarizes the light into one polarization state. After polarizing filter 106, the light passes into polarization switch 108 which actively switches the light into two orthogonal polarization states in synchronization with projector 102 displaying left and right eye images. The light then passes to polarization-preserving screen 110 to form projected image 112 on polarization-preserving screen 110. In this system, the images for one eye are displayed with one orthogonal polarization state while the images for the other eye are displayed with the other orthogonal polarization state.

FIG. 2 shows a stereoscopic display system with a polarization-switching light source. Polarization-switching light source 200 illuminates polarization-preserving projector 202. Polarization-preserving projector 202 projects an image with lens 204. Polarization-switching light source 200 switches the light into two orthogonal polarization states in synchronization with polarization-preserving projector 202 displaying left and right eye images. Polarization-preserving projector 202 and polarization-preserving lens 204 illuminate polarization-preserving screen 210 to form projected image 212 on polarization-preserving screen 210. As in the system of FIG. 1, the images for one eye are displayed with one orthogonal polarization state while the images for the other eye are displayed with the other orthogonal polarization state.

FIG. 3 shows the details of a polarization-preserving projector design. Light from polarization-switching light source 320 enters lens system 300. Lens system 300 passes the light to total internal reflection (TIR) prism 322 which consists of first TIR subprism 302 and second TIR subprism 312. The light enters first TIR subprism 302 and reflects off the interface between first TIR subprism 302 and second TIR subprism 312. Then the light exits first TIR subprism 302 and enters Philips prism 324 which consists of first Philips subprism 304, second Philips subprism 306, and third Philips subprism 308. The light enters first Philips subprism 304 and passes to the interface between first Philips subprism 304 and second Philips subprism 306. At the interface between first Philips subprism 304 and second Philips subprism 306, the blue wavelength region of the light is reflected and the green and red wavelength regions of the light are transmitted. The blue light reflects off the surface of first Philips subprism 304, then reflects from blue polarization-preserving SLM 316, then reflects off the surface of first Philips subprism 304, then reflects off the interface between first Philips subprism 304 and second Philips subprism 306 to rejoin the main beam to exit from Philips prism 324. The green light passes into second Philips subprism 306, then into third Philips subprism 308, then reflects from green polarization-preserving SLM 310, then rejoins the main beam to exit from Philips prism 324. The red light reflects from the interface between second Philips subprism 306 and third Philips subprism 308, reflects off the surface of second Philips subprism 306, then reflects from red polarization-preserving SLM 318, then reflects from the interface of first Philips subprism 304 and second Philips subprism 306, then reflects from the interface of second Philips subprism 306 and third Philips subprism 308 to join the main beam and to exit from Philips prism 324. The beam with all three colors modulated by polarization-preserving SLMs 310, 316, and 318 passes again through TIR prism 322 and then passes through polarization-preserving lens 314 to form a projected digital image.

FIG. 4 shows a polarization-switching light source. Linearly polarized light source 400 forms linearly polarized light beam 402. Linearly polarized light beam 402 passes through quarter wave plate 404 which produces right-hand circularly polarized light beam 406. Right-hand circularly polarized light beam 406 passes through polarization switch 418 which consists of dummy plate 408, half wave plate 412, rotor 414, and motor 416. Dummy plate 408 and half wave plate 412 form rotating disk 420 which spins around rotor 414 and is powered by motor 416. Right-hand circularly polarized light beam 406 passes alternately through dummy plate 408 and half wave plate 412 as they spin around rotor 414. When right-hand circularly polarized light beam 406 passes through dummy plate 408, there is no effect on the polarization and beam 410 is still right-hand circularly polarized. When right-hand circularly polarized light beam 406 passes through plate 412, the polarization of beam 410 is changed to become left-hand circularly polarized. Polarization switch 418 is synchronized with a projector to produce left-eye images polarized with one circular polarization state (e.g. right-hand) and right eye images with the orthogonal circular polarization state (e.g. left-hand). Alternately, quarter wave plate 404 may be arranged to produce left-hand circularly polarized light in which case half wave plate 412 changes the polarization to right-hand circularly polarized light.

FIG. 5 shows a front view of rotating disk 420 in polarization switch 418. Disk 420 rotates clockwise as shown by arrow 502. Right-hand circularly polarized light beam 406 passes through disk 420 at the position shown by arrow 500. Alternately, the disk may rotate counterclockwise.

FIG. 6 shows a method of stereoscopic projection. In step 600, polarization-switched light is generated. In step 602, the polarization switched light is modulated to make a left-eye digital image and a right-eye digital image. In step 604, the left-eye digital image and the right-eye digital image are projected for viewing. The modulation in step 602 may be in synchronization with the polarization of the polarization-switched light so that the image for the left eye is polarized in one state, and the image for the right eye is polarized in the orthogonal state.

Polarization-switching light sources may be constructed by polarizing a naturally unpolarized source of light such as an arc lamp. In this case, the polarizer may be an absorptive polarizer such as polarizing film that absorbs the unwanted polarization of light. A more efficient system makes use of polarization recovery to repolarize the unwanted polarization state so that there is more light in the desired polarization state. Another method is to start with an inherently polarized light source such as a polarized laser that may be a solid state laser, diode pumped solid state (DPSS) laser, gas laser, or optical parametric oscillator (OPO).

A polarization switch may be used to actively change the polarization state of the polarization-switching light source. A polarization switch may be mechanical such as the one shown in FIG. 4, or it may be an electro-optical or magneto-optical device such as a polarization cell based on the Pockels effect, Faraday effect, Kerr effect, or any other polarization-controlling optical element. The polarization switch may be built into the light source or it may be a separate element immediately after the light source which operates on light emitted by the light source in which case the light source and polarization switch together are considered to be a polarization-switching light source. When utilized for stereoscopic projection, the polarization switch should not spend significant time switching between states because the time between states may contribute to ghosting.

Wave plates may be also used to change polarization states of light. Achromatic wave plates make the same change in polarization across all wavelengths in a certain design region. When held at the proper rotational orientation relative to the beam direction of propagation, quarter wave plates make a 90 degree phase difference in the horizontal and vertical electric field vectors such that linearly polarized light is converted to circularly polarized light and vice versa. Half wave plates make a 180 degree phase difference such that linearly or circularly polarized light is converted to the orthogonal polarization state. Wave plates may be made of birefringent crystals cut as a specific orientation, or they may be made from birefringent plastic film with specific retardation in each axis. Achromatic wave plates may be made from a stack of multiple layers of plastic film at specific orientations such as those manufactured by ColorLink Inc. (Boulder, Colo.).

A polarization-preserving projector may be constructed by using optical components that are themselves polarization preserving. If all the lenses, prisms, SLMs, minors and other optical components of the projector are polarization preserving, the overall projector will be polarization preserving. Optical components are polarization preserving if they cause an acceptably low level of polarization changes for two orthogonal polarization states. Optical effects which can cause polarization changes include scattering, retardation, and polarization splitting. Scattering is an inherently depolarizing process. Retardation may occur from randomly distributed birefringence regions in plastic optical elements. Polarization splitting is caused at optical surfaces by differences in reflected or transmitted intensities of light that are of two different polarization states. Optical surfaces with a very high transmission or reflection throughout the wavelengths of operation generally have minimal polarization splitting. Certain antireflection (AR) coatings and all total internal reflection surfaces have minimal polarization splitting as long as the wavelengths and angles fall within their designed range of operation. Also, most coatings and materials have minimal polarization splitting when the angle of incidence (AOI) is low. The AOI for a ray of light incident on a surface is defined as the angle between the incident ray of light and the perpendicular to the surface. If the AOI is zero degrees, mirrors and transmissive surfaces have zero polarization splitting. If the AOI is 5 degrees, aluminum minors have approximately 0.1% polarization splitting and typical AR coatings have approximately 0.01%. If the AOI is 10 degrees, aluminum minors have approximately 0.2% polarization splitting and typical AR coatings have approximately 0.05% in the photopically significant middle of the visible region of light which extends from approximately 500 nm to approximately 600 nm.

Optical elements constructed of high quality optical glass are generally inherently polarization preserving in the bulk of the material because the index of refraction is uniform throughout. Plastic optical elements, on the other hand, sometimes have retardation which leads to depolarizing or non-uniform polarization if the index of refraction varies throughout the plastic material.

Polarization preserving SLMs may be designed using digital micromirror devices (DMDs) such as those available from Texas Instruments (Dallas, Tex.). Since the mirrors of a DMD are coated with a highly-reflecting material such as aluminum, the polarization splitting is low at small angles of operation which are typically 5 to 10 degrees AOI. If lower polarization splitting is desired, higher reflection coatings or other coatings with reduced polarization splitting may be used on the mirrors. In addition, AR coatings on the DMD window may help reduce polarization splitting of the DMD.

Glass TIR prisms may be designed to be polarization preserving. The interface between the two subprisms of a TIR prism has an air gap with AR coatings on both of the surfaces that form the air gap. In the example of FIG. 3, the AOI at the point where the beam passes through the air gap and AR coatings is 36 degrees. The transmission through the AR-coated surfaces of the TIR prism may have low polarization splitting if the AR-coating is designed to minimize polarization splitting. Between 500 and 600 nm, the polarization splitting may be less than 0.2%.

Philips prisms may be used for color splitting and recombining as described in detail in U.S. Pat. No. 3,659,918 the complete disclosure of which is incorporated herein by reference. Philips prisms with low polarization splitting may be constructed as described in “Design of Non-Polarizing Color Splitting Filters used for Projection Display System” by W. Chen et al., Displays, Elsevier, 2005, the complete text of which is incorporated herein by reference. The Philips prisms used by Chen have coatings designed with standard techniques of optical thin film design so that polarization splitting is reduced to near zero for all wavelengths of operation.

Polarization-preserving front-projection screens are commercially available with matte metallic coatings that diffusely reflect light while maintaining the polarization state with low depolarization. These screens are commonly used with polarization-based stereoscopic projection systems. Polarization-preserving rear projection screens are also available for use in rear projection systems.

Instead of using a projector that is polarization preserving, a projector that makes a known change in polarization states may be used as long as optical elements are included that perform compensating polarization adjustments after the light passes through the projector or internally in the projector so that the left and right eye images are still orthogonal. ColorLink filters may be helpful compensation elements for this purpose because they may be designed to change the polarization of different colors by different amounts. For example, if the polarization state of a specific color is rotated because of polarization splitting in the Philips prism, the polarization of that color can be corrected by rotating that color back to the desired polarization state by adding a ColorLink filter at the output of the projector.

An advantage of using a polarized-laser light source with a polarization-preserving projector is the high efficiency compared to systems that start with an unpolarized light source. With DMD light valves, typical polarized projection systems lose at least 50% of the light when using an unpolarized light source. Even with polarization recovery, 20% of the light is generally lost when converting from unpolarized to polarized light.

As opposed to SLM-based projectors, scanning projectors do not use an SLM. Instead they have a spot or line of light that is scanned over the area of the screen to form an image. Scanning projectors are typically based on laser light sources because the high collimation of the laser beam allows the projector to form a small spot at a distance. When compared to scanning projectors, SLM-based projectors typically have advantages in construction simplicity, alignment stability, and safety due to lower peak beam intensity.

Other implementations are also within the scope of the following claims. 

What is claimed is:
 1. A stereoscopic display system comprising: a polarization-switching light source characterized by a polarization state; and a polarization-preserving projector which is illuminated by the polarization-switching light source.
 2. The system of claim 1 wherein the polarization-preserving projector forms a left-eye digital image and a right-eye digital image, and the polarization state is changed in synchronization with an alternating projection of the left-eye digital image and the right-eye digital image.
 3. The system of claim 2 wherein an alternating projection of the left-eye digital image and the right-eye digital image forms a moving image.
 4. The system of claim 1 wherein the polarization state of the left-eye digital image is orthogonal to the polarization state of the right-eye digital image.
 5. The system of claim 4 wherein the polarization state of the left-eye digital image is a circular polarization state.
 6. The system of claim 1 further comprising: a polarization-preserving spatial light modulator in the projector.
 7. The system of claim 6 wherein the polarization-preserving spatial light modulator is a digital micromirror device.
 8. The system of claim 1 further comprising: a Philips prism in the projector.
 9. The system of claim 8 wherein the Philips prism comprises a color-splitting filter with low polarization splitting.
 10. The system of claim 1 wherein the polarization-switching light source comprises a laser.
 11. The system of claim 1 wherein the polarization-switching light source comprises a rotating disk that switches the polarization state.
 12. The system of claim 11 wherein the rotating disk comprises a wave plate.
 13. The system of claim 1 wherein the polarization-switching light source comprises an electro-optical device that switches the polarization state.
 14. The system of claim 1 wherein the polarization-preserving projector consists of polarization-preserving components.
 15. The system of claim 1 wherein the polarization-preserving projector is a front projector.
 16. The system of claim 1 further comprising: a polarization-preserving screen.
 17. A stereoscopic display system comprising: a polarization-switching light source characterized by an original polarization state; a projector which is illuminated by the polarization-switching light source; and a polarization compensation element; wherein the polarization compensation element compensates for a polarization change in the projector.
 18. A method of stereoscopic projection comprising: generating a light which is polarization switched; modulating the light to make a left-eye digital image and a right-eye digital image; and projecting the left-eye digital image and the right-eye digital image alternately to form a projected left-eye digital image and a projected right-eye digital image.
 19. The method of claim 18 wherein the modulating is synchronized with a polarization state of the light.
 20. The method of claim 18 wherein the projected left-eye digital image has one polarization state, and the projected right-eye digital image has an orthogonal polarization state.
 21. The method of claim 18 wherein the light is laser light. 